The present invention relates to a writing start position adjustment mechanism employed in a printer for adjusting a printing start position on a recording medium.
Conventionally, various printers are known such as ones utilizing a heat sensitive transfer or, an electrophotographic system (laser beam printer etc.) and the like as a printer for printing characters or figures outputted from a computer or word processor.
Among them is a printer which prints figures (or prints characters) on a continuous form recording medium, having feed holes, referred to as a so-called fanfold paper (hereinafter, simply abbreviated as a continuous sheet).
The continuous sheet has a perforated tear line at each of the folded portions thereof, so that it can easily be cut off.
The continuous sheet on which the image has been printed is cut off along the perforated tear line for use in accordance with its need. Thus, a printer for printing images on the continuous sheet is preferably arranged in such a manner that it recognizes the perforated tear line by some methods, and starts printing at a position spaced apart from the perforated tear line by a predetermined distance.
FIG. 4 shows an example of a laser beam printer for printing images on a continuous sheet utilizing an electrophotographic image forming process.
The laser beam printer has a toner cleaner 2, a discharging unit 3, charging unit 4, scanning optical system 5 for introducing a scanning laser beam onto a photoconductive drum 1, developing unit 6, and a transferring unit 7 disposed, respectively, around the photoconductive drum 1 along the rotational direction thereof in a predetermined order, and further, a fixing unit 8 is disposed at a position toward which the continuous sheet 9 is fed, and a tractor 10 for feeding and driving the continuous sheet 9 is disposed in the feed path through which the continuous sheet 9 is fed from the photoconductive drum 1 to the fixing unit 8.
The surface of the photoconductive drum 1 is scanned (exposed) by a laser beam emitted from the scanning optical system 5 in the axial direction of the photoconductive drum 1 (main scanning). The photoconductive drum 1 is rotated (auxiliary scanning), and a latent image is formed on the circumferential surface of the photoconductive drum 1. The latent image is developed by the development unit 6 forming a toner image, and the toner image is transferred onto the continuous sheet 9 by the transfer unit 7. Then the transferred toner image is fixed on the continuous sheet 9 by the fixing unit 8, after which the continuous sheet is ejected.
The tractor 10 is arranged in such a manner that a pair of endless belts 11, each of which has projections engaging the feed holes, defined along respective side edge of the continuous sheet 9 at predetermined intervals, are rotatably stretched between two pulleys 12, 13 with the circulating track on the upper side thereof coinciding with the feed path of the continuous paper 9. These endless belts 11 are disposed in parallel to each other in correspondence to the feed holes defined along the opposite side edges of the continuous sheet 9.
One of the pulleys 13 is connected to a not shown motor as a driving means so that the endless belts 11 are driven at a speed synchronously with the peripheral speed of the photoconductive drum 1. Further, a feed position sensing mechanism 20 is connected to the other pulley 12.
The feed position sensing mechanism 20 comprises a disc plate 21 having slits 21A defined from the center to the outer circumference thereof, and rotated synchronously with the drive of the tractor 10, and a photosensor 22. The photosensor 22 includes a light emitting member and light receiving member. The light emitting member of the photosensor 22 and the light receiving member of the photosensor 22 are oppositely arranged defining a sensing region through which the slits 21A of the disc plate 21 pass upon rotation of the disc plate 21. Thus, a pulse signal is outputted each time the slits 21A pass through the sensing region of the photosensor 22. As described above, since the disc plate 21 rotates synchronously with the rotation of the tractor 10, the pulse signal is outputted synchronously with the drive of the tractor 10, i.e., the feeding amount of the continuous sheet 9.
A leading edge sensing mechanism 30 for sensing the leading edge of the continuous sheet 9 is provided with the feed path of the continuous sheet 9 extending from the tractor 10 to the fixing unit 8.
The leading edge sensing mechanism 30 is arranged in such a manner that an end of a swingable lever 31 is disposed at the position where it interferes with the continuous sheet feed path. A photosensor 32 is provided to sense the other end of the lever 31 on any one side of the swinging range thereof (in the figure, the photosensor senses the leading edge of the continuous sheet 9 when the lever 31 is kicked by the continuous sheet 9, i.e. when a signal is interrupted). When the continuous sheet 9 is located at the position corresponding to the leading edge sensing mechanism 30, the lever 31 is operated to rock by the continuous sheet 9. A signal from the photosensor is interrupted thereby, so that it is sensed that the continuous sheet 9 is present at the position.
In the above arrangement, when the continuous paper 9 is loaded in the laser beam printer, it is set to the tractor 10 with the leading edge thereof located between the tractor 10 and the leading edge sensing mechanism 30. Then, the continuous sheet 9 is fed, and thus, the position of the continuous sheet 9 is recognized through the leading edge sensing mechanism 30 which senses the leading edge of the continuous sheet 9. Thereafter images can be printed from the predetermined position (printing start position) of the continuous paper by counting signals from the feed amount sensing mechanism 20.
More specifically, when the continuous sheet 9 is set, the leading edge position thereof is detected. Thereafter the printing of the images is carried out by recognizing the position of the continuous sheet 9 (the position of the perforated tear line) based on the position of the feed holes thereof (based on the signals from the feed position sensing mechanism 20).
Nevertheless, since the positional accuracy of the feed holes of the continuous sheet is not so high, and further, the sheet is elongated or shortened by the circumferential environments such as humidity and the like, when images are printed depending upon the leading edge of the continuous sheet 9 and the position of the feed holes, a problem arises in that positions from which the images are printed vary in the sheet feed direction. This problem may be overcome to some degree by accurately detecting the position of the continuous sheet, but since mechanical errors of parts may be accumulated, it is very difficult to greatly improve the sensing accuracy and thus the improvement thereof has been desired.